1. Field of the Invention
The present invention relates to a variable-displacement vane pump for use in an apparatus using pressurized fluid, such as a power steering unit for reducing force required to steer a steering wheel of an automobile.
2. Description of the Related Art
As a pump for a power steering unit, a positive-displacement vane pump has usually been employed which is directly operated by an engine of an automobile. The discharge flow rate of the foregoing positive displacement pump is changed to correspond to the number of revolutions of the engine. Therefore, the positive displacement pump has a characteristic which is mutually contradictory to auxiliary steering force which must be provided for the power steering unit. The auxiliary steering force must be enlarged when the automobile is stopped or the automobile runs at low speed and reduced when the automobile runs at high speed. Therefore, the positive displacement pump must have a large capacity which enables a discharge flow rate to be maintained with which required auxiliary steering force can be obtained even if the automobile runs at low speed with a small number of revolutions of the engine. Moreover, a flow control valve must be provided which controls the discharge flow rate to be not larger than a predetermined quantity when the automobile runs at high speed with a large number of revolutions of the engine. Therefore, the positive displacement pump involves an increase in the required elements, a complicated overall structure and a complicated structure of passages. Thus, the overall size and costs cannot be reduced.
To solve the problems experienced with the above-mentioned positive displacement pump, variable-displacement vane pumps each of which is capable of reducing a discharge flow rate per revolution (cam cc/rev) in proportion to an increase in the number of revolutions have been disclosed. For example, variable-displacement vane pumps of the foregoing type have been disclosed in Japanese Patent Laid-Open No. 53-130505, Japanese Patent Laid-Open No. 56-143383, Japanese Patent Laid-Open No. 58-93978, Japanese Utility-Model Publication No. 63-1478, and Japanese Patent Laid-Open No. 7-243385. The foregoing variable displacement pumps do not need the flow control valve of the capacity type. Moreover, the variable displacement pump exhibits an excellent energy efficiency because waste of drive horsepower can be eliminated. Since a return to a tank can be prevented, a rise in the temperature of oil can be prevented, a rise in the temperature of oil can be prevented. Moreover, problems of leakage in the pump and deterioration in the capacity efficiency can be prevented.
An example of the foregoing variable-displacement vane pump will simply be described with reference to FIG. 16 which shows the structure of the pump disclosed in Japanese Patent Laid-Open No. 7-243385. Referring to FIG. 16, reference numeral 1 represents a pump body, 1a represents an adapter ring and 2 represents a cam ring provided in an elliptic space 1b formed in the adapter ring 1a of the body 1, the cam ring 2 being swingably supported through a support shaft portion 2a which serves as a fulcrum for a swinging operation. The cam ring 2 is urged by an urging means (compression coil spring) for urging the cam ring 2 in a direction indicated by a hollow arrow F show in FIG. 16.
Reference numeral 3 represents a rotor eccentrically accommodated at a position adjacent to an end in the cam ring 2 in such a manner that a pump chamber 4 is formed at another end. Since the rotor 3 is rotated by an external power source, the rotor 3 forwards/rearwards moves a vane 3a which is held such that the vane 3a is able to move in the radial direction. Reference numeral 3b represents a drive shaft for the rotor 3. The rotor 3 is rotated in a direction indicated by an arrow shown in FIG. 16.
Reference numerals 5 and 6 represent fluid-pressure chambers formed in a pair on the two outer sides of the cam ring 2, the fluid-pressure chambers 5 and 6 being arranged to serve as high and low pressure portions in the elliptic space 1b of the adapter ring 1a of the body 1. In the chambers 5 and 6, passages 5a and 6a for introducing fluid pressures across a variable metering orifice 12 provided for a pump discharge-side passage 11 for controlling the swinging operation of the cam ring 2 are opened through a spool-type control valve 10 to be described later. When the fluid pressures across the variable metering orifice 12 in the pump discharge-side passage 11 are introduced through the passages 5a and 6a, the cam ring 2 is swung to a required direction. Thus, the capacity in the pump chamber 4 is varied so that the discharge flow rate is controlled to correspond to a flow rate in the discharge portion of the pump. That is, the flow rate in the discharge portion is controlled in such a manner that the flow rate in the discharge portion is reduced in inverse proportion to enlargement of the number of revolutions of the pump.
Reference numeral 7 represents an opening (a suction port) in the suction portion of the pump, the opening 7 being opened to face a pump suction-side region 4A of the pump chamber 4. Reference numeral 8 represents an opening (a discharge port) in the pump discharge portion, the opening 8 being opened to face a pump discharge-side region 4B of the pump chamber 4. The openings 7 and 8 are provided for either of a pressure plate or a side plate (not shown), the plates being securing walls for holding a pump element incorporating the rotor 3 and the cam ring 2 from two side portions.
The cam ring 2 is urged by the compression coil spring from the fluid-pressure chamber 6, as indicated with symbol F shown in the drawing. The cam ring 2 is pressed in a direction in which the capacity in the pump chamber 4 is maximized. Reference numeral 2b shown in the drawing represents a sealing member provided on the outer surface of the cam ring 2 so as to define the fluid-pressure chambers 5 and 6 in association with a bearing portion 2a, the chambers 5 and 6 being defined on the right-hand and left-hand portions in the pump chamber 4.
Reference numerals 7a and 8a represent whisker-like notches formed continuously from ends of the opening 7 in the pump suction portion and the opening 8 in the pump discharge portion. When a pumping operation is performed by rotating the rotor 3 so that the leading end of each vane 3a is slid on the inner surface of the cam ring 2, the notches 7a and 8a gradually relieve the fluid pressure from the high pressure portion to the low pressure portion in a region from a space adjacent to the ends of the openings 7 and 8 and held between the vanes to a space between the vanes adjacent to the foregoing space. Thus, surge pressure and pulsation are prevented.
The spool-type control valve 10 is operated by dint of different pressures P1 and P2 across a variable metering orifice 12 disposed at an intermediate position of the pump discharge-side passage 11. When fluid pressure P3 corresponding to the flow rate in the discharge portion of the pump is introduced into the fluid-pressure chamber 5 at a position on the outside of the cam ring 2, a sufficiently high flow rate can be maintained in the initial stage of the operation of the pump. In particular, in a state where the different pressure across the variable orifice 12 is raised to be a level not lower than a predetermined level when a load is applied because of the operation of the apparatus using the fluid pressure, the control valve 10 introduces the fluid pressure P1 upstream of the variable orifice 12 into the high-pressure-side fluid-pressure chamber 5 on the outside of the cam ring 2, the fluid pressure P1 being introduced as control pressure. Thus, any swing of the cam ring 2 can be prevented.
The variable-displacement vane pump having the above-mentioned structure incorporates elements, for example, the body 1, each having a complicated structure. What is worse, a large number of elements must be provided. Thus, there arises a problem in that each element cannot easily be machined and assembled. Moreover, the size and weight of the pump cannot easily be reduced. Thus, the foregoing pump is susceptible to improvement.
The conventional variable displacement pump has a structure that a pressure plate is disposed to an end of the rotor 3 and the cam ring 2 which constitute the pump chamber 4 in the body 1. A discharge chamber into which pressurized oil discharged from the pump chamber is introduced is formed on the backside of the pressure plate. The discharged pressure from the discharge chamber of the pump causes the pressure plate to be brought into contact with the cam ring 2 and the rotor 3 under a predetermined pressure. Thus, the pump chamber 4 is formed between an end surface of a portion of the side plate or the pump body 1 disposed opposite to the cam ring 2 and the rotor 3. As a result, hydraulic oil can be sucked into the pump chamber 4 and discharged from the same.
The above-mentioned variable displacement pump is different from a usual positive displacement vane pump in that the pump chamber 4 is composed of a pump suction-side region 4A and a pump discharge-side region 4B which are disposed at asymmetric positions with respect to the rotational shaft 3b. On the other hand, an annular recess is formed on the backside of the foregoing pressure plate except for the portion including the rotational shaft 3b. The foregoing recess is formed into a discharge chamber into which pressurized oil discharged from the discharge portion of the pump is introduced.
Therefore, the discharged pressure from the inside portion of the discharge chamber acts on substantially the overall back surface of the pressure plate because of the annular recess. Since the pump suction-side region 4A of the pump chamber 4 is formed eccentrically, great force for pressing the pressure plate against the pump chamber 4 acts on the above-mentioned portion. If the eccentric force acts on the plate, the portion is deformed. As a result, there is apprehension that the plate is excessively pressed against the cam ring 2 and the rotor 3.
Therefore, the pressure plate is required to have rigidity capable of during the eccentric force.
If the pressure plate is deformed, a gap is formed between the cam ring 2 and the rotor 3. What is worse, an amount of internal leakage of pressurized oil is enlarged when the pressure is high. Therefore, a countermeasure must be taken.
As described above, the above-mentioned variable displacement pump is required to have a completely modified overall structure, to enable the structures of the elements to be simplified, the number of the elements to be reduced, the machining and assembling processes to be performed easily, reliability of the operation of the pump to be improved and the size, weight and cost of the pump to be reduced.